Field of the Invention
The invention relates to an input amplifier for input signals with steep edges (in particular "high-low" edges) which has at least one transistor with an electrode connected to an output.
CMOS input amplifiers have been known for a relatively long time. They are used for the most manifold purposes in circuitry. For instance, an input amplifier of the above type is described in the textbook "CMOS Analog Circuit Design" by P. E. Allen and D. R. Holberg, p. 381.
A prior art amplifier of that type is illustrated in FIG. 1, which will be described in detail below.
Possible uses of such prior art input amplifiers are for instance LVTTL circuits (LVTTL stands for low voltage transistor-to-transistor logic) and SSTL logic circuits (SSTL stands for stub series terminated logic). In LVTTL logic circuits, leading and trailing voltage edges of approximately 0.8 V to 2.0 V occur, while the SSTL logic circuits have corresponding leading and trailing edges of approximately 400 mV about a reference value.
In both logic circuits, that is, the LVTTL logic circuits and the SSTL logic circuits, the trailing voltage edges are very steep, so that the input edges are correspondingly fast. If numerous input amplifiers are provided on a chip, then in a current-saving operating mode as many of them as possible should be deactivated so that they will not consume current. It is moreover desirable to put the input receivers that have remained active into a current-free state by means of the input signals. In any case, only those circuits that are being driven should actually be active.
However, it has been found that in the case of steep edges of the input signals, the input amplifier enters a current-free final state much too quickly, thereby preventing switching of the output signal.
This problem has been previously attacked by employing asymmetrical input amplifiers. This renders the switching operation fast enough so as to be concluded before the final state is reached. As a consequence of this dimensioning, however, in the least favorable case the DC current consumption is increased ("worst-case DC consumption").